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Selecting a voltage reference

Posted: 13 Mar 2014 ?? ?Print Version ?Bookmark and Share

Keywords:voltage reference? Temperature coefficient? Initial drift? Thermal hysteresis?

In analogue and mixed-signal circuits, the standard against which other signals are measured is typically the voltage reference. Inaccuracy and variation in the voltage reference directly impact the accuracy of the whole system. Let's look at how accuracy specifications and other criteria apply when selecting a voltage reference.

Initial accuracy indicates the variance of output voltage as measured at a given temperature, usually 25C. While the initial output voltage may vary from unit to unit, if it is constant for a given unit, then it can be easily calibrated.

Temperature drift is probably the most widely used spec to evaluate voltage reference performance, as it shows the change in output voltage over temperature. Temperature drift is caused by imperfections and non-linearities in the circuit elements. For many parts, the temperature drift specified in ppm/C, is the dominant error source. For parts with consistent drift, some calibration is possible.

A common misconception regarding temperature drift is that it is linear. But one should not assume that a reference will drift a lesser amount over a smaller temperature range. Temperature coefficient (TC) is generally specified with a "box method" to convey the likely error over the entire operating temperature range. It is calculated by dividing the min-max voltage difference over the entire temperature range, divided by the total temperature range (figure 1).

Figure 1: Voltage reference temperature characteristics.

These min and max voltage values may not occur at the temperature extremes, leading to regions where the TC is much larger than the average calculated for the entire specified temperature range. This is especially true for the most carefully tuned references, often identified by their very low temperature drift, where the linear drift components have been compensated leaving a residual non-linear TC.

The best use of the temperature drift specification is to calculate maximum total error over the specified temperature range. It is generally inadvisable to calculate errors over unspecified temperature ranges unless the temperature drift characteristics are well understood.

Long term stability is a measure of the tendency of a reference voltage to change over time, independent of other variables. Initial shifts are largely caused by changes in mechanical stress, from the difference in expansion rates of the lead frame, die and mould compound. This stress effect tends to have a large initial shift that reduces quickly with time.

Initial drift also includes changes in electrical characteristics of the circuit elements, including settling of device characteristics at the atomic level. Longer-term shifts are caused by electrical changes in the circuit elements, often referred to as "ageing." This drift tends to occur at a reduced rate compared to initial drift, and decreases over time. It is therefore often specified as drift/khr. Voltage references tend to age more quickly at higher temperatures.

Thermal hysteresis is an often-overlooked specification which can be a dominant source of error. It is mechanical in nature, and is the result of changing die stress due to thermal cycling. Hysteresis can be observed as a change in output voltage at a given temperature after a large temperature cycle. It is independent of temperature coefficient and time drift, and reduces the effectiveness of initial voltage calibration.

Most references tend to vary around a nominal output voltage during subsequent temperature cycles, so thermal hysteresis is usually limited to a predictable maximum value. Each manufacturer has their own method for specifying this parameter, so comparing typical values can be misleading. Distribution data, as provided in data sheets such as the LT6654 and LT6656, is far more useful when estimating output voltage error.

Recently a new class of voltage references has been introduced to the market. Housed in a hermetic surface mount package, these products exhibit significantly improved long term stability and thermal hysteresis performance when compared with the same products in traditional plastic SOT-23 and MSOP packages.

Other specifications
Additional specifications that may be important, depending on application requirements include:
???Voltage Noise
???Line Regulation/PSRR
???Load Regulation
???Dropout Voltage
???Supply Range
???Supply Current
???Reference types

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